The present invention relates to an improved material for covering the UV lamps in tanning beds.
From European Patent 0016870 there are already known plastics based on methyl methacrylate which have high transmission for UV A radiation (315 to 380 nm) and for UV B radiation (280 to 315 nm). Their high UV transmission is preserved even during prolonged exposure to radiation if the plastic contains a small concentration of a sterically hindered amine. Corresponding compounds trap radicals which are formed during radiation exposure and which would otherwise slowly destroy the plastic material. Such additives are described in Japanese Patent 03-47856, and are known as xe2x80x9chindered amine light stabilizersxe2x80x9d or HALS. Plastics which are based on polymethyl methacrylate (PMMA), which contain sterically hindered amines and which have high UV transmission are also described in German Unexamined Specification DE-OS 3421859.
Heretofore Plexiglas(copyright) GS 2458 (commercial product of Rohm GmbH) has been used to cover the UV lamps of tanning beds. The material is easy to care for and transmits a high percentage of tanning UV radiation. The thickness of the covers used has ranged between 3 and 8 mm.
Since both the radiant power of UV lamps on the whole and the UVB component of the radiation have increased in the course of technical development, a considerable reduction of transmission is suffered in standard materials with thicknesses of  greater than 4 mm after about 20 hours of exposure.
Exposure Results
In this case: standard formula (Plexiglas(copyright) 2458), 8 mm thick, tempered (30 minutes, 160xc2x0 C.). Lamp: light box, Ergoline ST Turbo Power lamps, 9 tubes:
A yellow discoloration known as yellow core develops in the interior of the plate. The UV light is absorbed, the customer no longer becomes tanned and the plate must be replaced relatively early.
Object
The object was therefore to provide a material which, even at the lamp intensities that are common today, yellows not at all or only slightly while also having high UV transmission. At the same time, it must be ensured that the additives which prevent development of the yellow core are physiologically safe. Only substances which are approved by the FDA and are in conformity with the European Union and German Health Agency directives may be added. It is intended that plates will be made available which do not exhibit yellow core even at the lamp powers which are standard today in combination with material thicknesses of more than 4 mm. The large thicknesses of more than 4 mm are necessary, since the cover of the UV lamps has cantilever structure and the plates are hot-formed at about 160xc2x0 C.
Achievement
Good UV transmission over a long period is a primary requirement for tanning bed material.
It has now been found that the object can be achieved by addition of alcohols, alcohol mixtures and even water and/or further compounds to the monomer mixture of the standard formula. The compound or the mixture respectively being used will be referred to hereinafter as active components. Examples of suitable alcohols are monohydric or polyhydric C1 to C10 alcohols, which may also be branched, such as methanol, ethanol, propanol, isopropanol or tert-butanol or pentanols. A further improvement can be achieved by the use of further compounds, such as vinyl compounds, butyl lactate and siloxanes as well as mixtures of the said compounds. Vinyl compounds can be understood as vinyl esters of aliphatic carboxylic acids, such as vinyl acetate. There can also be used vinyl esters of general formula I:
[see original for formula] (Formula I) 
where:
R1, R2 and R3 are alkyl groups, wherein at least one group represents a methyl group. The total number of carbon atoms in groups R1 to R3 is 9, 10 or 11.
The compounds of formula I are available from Shell Chemicals under the names VeoVa9 (9 carbon atoms, R1 to R3), VeoVa10 (10 carbon atoms, R1 to R3) or VeoVa11 (11 carbon atoms, R1 to R3).
The alcohols, alcohol mixtures or further active components are mixed in with the monomer mixture in proportions of 0.1 to 10 wt %. Contents of 0.2 to 8 wt % of active components are preferred, and contents of 0.2 to 6 wt % of active components are especially preferred.
Methyl methacrylate comprises 85 to 95 wt % of the monomer mixture.
Composition of the standard formula:
As initiator there can be used the initiators that are standard in cast polymerization; see H. Rauch-Puntigam, Th. Vxc3x6lker in xe2x80x9cAcrylic and Methacrylic Compoundsxe2x80x9d, Springer-Verlag 1967; Houben-Weyl, 4th Edition, Volume XIV/1, Macromolecular Substances, Part 1, pp. 1010 to 1078, Georg Thieme Verlag, 1961).
As plasticizer there can be used acetyl tributyl citrate or triethyl citrate. In some cases it may also be possible to omit the plasticizer.